FIG. 1 schematically illustrates an example of an embodiment of a radio transmitter 10. Transmitter 10 may, for example, be a radar transmitter or a transmitter of a mobile communication unit such as a mobile phone. In the shown example, transmitter 10 comprises an oscillator 12, a voltage amplifier 18, and a power amplifier 24.
The oscillator 12 may generate an oscillatory low power signal 14, 16. The low power signal 14, 16 may for example be a differential signal, e.g. provided by the difference between a first voltage 14 and a second voltage 16 output by the oscillator 12.
The voltage amplifier 18 may be coupled between the transmitter 12 and the power amplifier 24. The voltage amplifier 18 may for example be an individual amplifier or comprise an amplifier chain. The voltage amplifier 18 may amplify the low power signal 14, 16 to generate a corresponding low power signal 20, 22. The low power signal 20, 22 may be fed to the power amplifier 24. The power amplifier 24 may generate a high power signal 26, 28 according to the low power signal 20, 22 and, thus, according to the low power signal 14, 16. Here and throughout this application, “according to” means “as a function of at least” or “based at least on”. The high power signal 26, 28 may be fed for example to an antenna (not shown) to emit an electromagnetic wave, or to some other load (not shown). The oscillator 12 may comprise a phase-locked loop (PLL) (not shown) comprising, for example, a voltage controlled oscillator (VCO) (not shown).
Power amplifier 24 may be controlled by, and responsive to a power control signal 32. For example, power control signal 32 may be a binary signal, comprising first values and second values. The first and second values may, for example, be high and low values, respectively. For example, power amplifier 24 may be energized in response to the power control signal 32 assuming the first value and be energized (powered off) in response to the power control signal 32 assuming the second value. The power control signal 32 may be a pulse width modulated (PWM) signal. Power amplifier 24 may thus be operated for example at an average power that is e.g. proportional to a duty cycle of the power control signal 32. A duty cycle of zero may correspond to zero output power. A duty cycle of one may correspond to maximum output power. Alternatively, power control signal 32 may be an analogue signal.
The total power consumed by the power amplifier 24 may for example be a sum of e.g. a DC operating power and an AC output power. The DC operating power may be required to operate the internal electronics of the power amplifier. The AC output power may be transferred to the antenna or other load, if connected. For example, when the high power signal 26, 28 is not fed to any load, i.e. when no load is connected, the AC output power may be zero. The AC output power may also depend on the amplitude of the low power signal 20, 22. In contrast, the DC operating power consumed by the power amplifier 24 may be largely independent of the amplitude of the oscillatory signal (low power signal 20, 22 in the example) that is fed to the power amplifier 24. The DC operating power may be converted into thermal energy in the power amplifier 24. In other words, the power amplifier 24 may generate heat. The heat may be dissipated over the entire transmitter 10. In particular, a temperature of the oscillator 12 may rise. This may cause an undesired frequency drift, because the oscillation frequency of the oscillator 12 may generally depends on the temperature of the oscillator 12. In particular, semiconductor-based oscillators can be very sensitive to temperature. For example, a millimeter-wave VCO, e.g. a 77-GHz VCO, may have a frequency drift of −20 MHz per Kelvin (K). Although in many situations, thermal effects can be fairly slow compared to e.g. electrical phenomena, it has been observed that the oscillation frequency of the oscillator 12 may be affected even when the power amplifier 24 is switched on and off at relatively high frequencies, e.g. above 10 Hz or even above 1 kHz, e.g. when the power amplifier 24 is controlled using PWM. In other words, the oscillator 12 may be sensitive to temperature fluctuations that occur over short periods, e.g. shorter than a millisecond.
U.S. Pat. No. 6,815,643 B2 (Von der Ropp) describes a semiconductor device in which an integrated circuit executes dummy operating cycles in order to generate heat if the temperature of the semiconductor device drops below a lower limit value.